Hydrogen sulfide is a toxic, corrosive and malodorous compound. It may be found in a variety of liquid and gaseous media, such as natural gas, petroleum, refinery gas streams, carbon dioxide, hydrogen, coal gas streams, gas streams from viscose rayon production, tars and asphalt, shale gas, coke oven gases, ammonia synthesis gas, rubber vulcanization streams, gases from sulfurization plants, turpentine production, pulp and paper mill effluent, sewage, brine drilling mud, landfills, phosphoric acid production gas streams and other industrial gas streams and effluents. It is also found in the tail gases and liquids of some hydrogen sulfide scrubbing processes such as Claus plants and amine scrubbing units.
In part because hydrogen sulfide is a highly toxic, corrosive and malodorous chemical, its release into the environment is strictly regulated by the Environmental Protection Agency, the Department of Environmental Resources, and other regulatory agencies throughout the world. The release of hydrogen sulfide is also of concern because it has been linked to the formation of acid rain.
Methods for removing hydrogen sulfide from gaseous and liquid media including the sources listed above, may generally be classified as either regenerative and non-regenerative. Regenerative methods typically employ sulfur-scavenging compounds, and have the desirable feature that the scavenging compounds can be recycled. The regeneration of the sulfur-scavenging compounds reduces the cost, both economically and environmentally, associated with the sulfur removal process. For example, regeneratable sulfur-scavenging compounds which have reacted with hydrogen sulfide need not be disposed of as waste, but can be recycled back to an active form. In a preferred regenerative process, the sulfur from the hydrogen sulfide is also recovered in a useful form.
Various amines and alkanolamines, which may be regenerated, have been used to remove acids, such as hydrogen sulfide, from gas streams. U.S. Pat. No. 2,776,870 discloses that aqueous amines and alkanolamines are useful for removing acids from a gaseous mixture. Hydrogen sulfide may be selectively removed from gas streams containing carbon dioxide by use of triethanolamine or methyldiethanolamine.
British Published Patent Specification No. 2103645 discloses that hydrogen sulfide and carbon dioxide may be removed from a gas mixture by contacting the mixture with a solvent comprising a tertiary amine and a physical absorbent. Suitable physical adsorbents include N-methylpyrrolidone and sulfolane.
U.S. Pat. No. 4,112,051 discloses a process for removing acidic gases from a gaseous mixture with an amine-solvent liquid absorbent comprising (1) an amine comprised of at least about 50 mole percent of a sterically hindered amine; and (2) a solvent for the amine mixture which is also a physical absorbent for the acid gases. Suitable sterically hindered amines include various piperidine compounds. Suitable solvents include sulfones, pyrrolidone and piperidine compounds, to name a few.
U.S. Pat. No. 4,978,512 discloses methods for reducing the levels of hydrogen sulfide and organic sulfides in a hydrocarbon stream by contacting the stream with a composition comprising the reaction products of a lower alkanolamine with a lower aldehyde. The reaction products include triazine and/or bisoxazolidine compounds.
U.S. Pat. No. 4,647,397 discloses a process and composition for removing hydrogen sulfide and similar sulfides from a gas stream. The gas stream is contacted with a substituted aromatic nitrile having an electron-attracting substituent on the aromatic ring, where the substituent has electron-attracting ability at least equal to that of halogen, and an organic tertiary amine, in an inert organic solvent, such as N-methyl-2-pyrrolidone. The spent contacting solution may be regenerated by heating the solution above the decomposition temperature of the reaction products to separate the sulfides from the liquid phase absorbent solution.
U.S. Pat. No. 4,775,519 discloses a continuous process for removing acid gas components from a gas stream by counter-currently contacting the stream with an aqueous solution of a mixture of N-methyldiethanolamine (MDEA) with imidazole or a methyl substituted imidazole. The gas is de-absorbed from the MDEA and the imidazole by reducing the pressure and causing the gas to flash.
U.S. Pat. No. 4,624,838 discloses a process for removing acid gases from a gaseous stream by contacting the stream with an aqueous scrubbing solution containing a hetero nitrogen compound comprising either a five- or six- membered ring having a pK.sub.a no greater than about 8. Preferred hetero nitrogen compounds include imidazole and piperazine compounds.
U.S. Pat. No. 5,128,049 discloses a method for reducing the hydrogen sulfide content of hydrocarbon-containing fluids and aqueous solutions by injections of a dilute solution of a scavenging agent. Suitable scavenging agents include hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine and various other compounds.
As described in our copending U.S. patent application Ser. No. 08/026,891, now U.S. Pat. No. 5,347,003, sodium sulfides may be formed from a hydrogen sulfide-containing stream and a sodium hydroxide solution. However, it has been observed that if any carbon dioxide is also present in the stream, some of the sodium sulfides may react with the carbon dioxide and regenerate hydrogen sulfide. Carbon dioxide presence in hydrogen sulfide containing streams is not uncommon, and presents significant problems for regenerative methods directed to scrubbing hydrogen sulfide from liquid and gaseous streams.
There is a need in the art for commercially viable methodology which can scavenge hydrogen sulfide from liquid and gaseous streams containing both hydrogen sulfide and carbon dioxide. An even greater benefit may be realized if the hydrogen sulfide is converted to a commercially useful form. The preparation of sulfur compounds in such form provides a further financial incentive for the scavenging of hydrogen sulfide and even further reduces the burden on waste disposal systems.